1. Technical Field
The present invention relates to radio transmitters and, more specifically, to the final power amplifier stage of high frequency (HF) radio transmitters.
When a transmitter is required to transmit a signal which contains components at several distinct frequencies, non-linearities in the final power amplifier give rise to intermodulation products (IMPs) at the output of the amplifier at frequencies other than those present in the input signal. The IMPs therefore distort the output and hence cause interference in adjacent channels. This effect gives rise to a significant technical problem where the frequency of the input signal to the final power amplifier stage represents information, for example if the input signal is the output of a parallel tone modem, or where the transmitter is being used to transmit several different signals at the same carrier frequency, as in an independent sideband mode of operation. For such purposes, a typical specification requires that any IMPs should be reduced by at least -35 dB relative to the wanted signals. Although it is possible to achieve this with amplifiers using valves, it has hitherto been difficult and impractical to produce IMPs reduced by more than -30 dB with solid state class B linear amplifiers operating at radio frequencies.
2. Prior Art
Typically, attempts have been made to reduce distortion in such amplifiers by the use of inverted feedback techniques and a number of amplifier stages, in order to compensate for the reduction in gain produced by the negative feedback. Improvements may also be obtained by carefully selecting the biasing conditions of the transistors used. Such design considerations are well discussed in the literature. However, such techniques have been found ineffective in meeting the high specifications required.